Advances in the efficiency of both spark-ignition and compression-ignition internal combustion engines have contributed significantly to improved fuel economy of vehicles. Despite these improvements in engine efficiency an appreciable fraction of the available energy in the fuel is discharged as waste heat, a sizeable portion of which is discharged at high temperature in the vehicle exhaust.
Yet further increases in vehicle fuel economy could be achieved if at least a portion of the waste heat contained in the exhaust stream could be efficiently captured.